Instapaper has an interesting take on username. It allows you to create an account with either a simple username (mynamehere) or an email address. However, it strongly recommend that you use an email address as the username because it does not separately store an additional email address field in your account. Instapaper has this setup partly because it offers a very long barrier to initial user. To "register for an account", you just enter a single username or email address. You can immediately start using the system. You do not have to set a password. Isn't this clever?
You can setup a password if you want to, and it's probably a good idea since it would be easy to guess people's usernames.